The present invention relates to a motor controller and an electric power steering apparatus.
In many cases, conventional motor controllers of electric power steering apparatuses (EPS) include anomaly detecting means. The anomaly detecting means detects an anomaly when flow of electric current fails in any one of U, V, and W phases, due to a break in power supply lines or damage to contacts of a driver circuit. When such an anomaly is detected, control of operation of a motor is quickly stopped and thus fail safe is performed.
However, in the EPS, if the control of the operation of the motor is suspended, the steering characteristics are greatly changed. Specifically, increased steering force becomes necessary for the driver to accurately manipulate a steering wheel. In this regard, Japanese Laid-Open Patent Publication No. 2003-26020, for example, discloses a motor controller that, even if a failure of electric current flow is detected in a certain phase, continuously controls the operation of a motor by employing phases free of failure of electric current flow as electric current flowing phases. In this case, although torque ripple is caused due to decrease of the number of the electric current flowing phases, assist force is continuously applied to a steering system, thus preventing increase of load on the driver caused in the fail safe.
When the steering wheel is manipulated rapidly, the motor of the EPS must be rotated at a high speed exceeding a base speed. In this case, response to such rapid steering is ensured by carrying out field weakening control.
When used in an EPS for which improved silence is required, a motor is driven usually by supply of sine wave electric current. In many of such cases, detected phase electric current values are converted into d-axis and q-axis electric currents of a d/q coordinate system. Feedback control is then performed in the d/q coordinate system in order to generate and output a motor control signal for carrying out the supply of the sine wave electric current. Field weakening control is carried out by setting a d-axis electric current command value of the feedback control to a negative value corresponding to rotation angular velocity of the motor. In other words, using demagnetizing magnetomotive force produced by counteraction of a d-axis armature caused by a d-axis electric current flowing in a negative direction, magnetic flux in a d-axis direction including permanent magnetic flux is decreased. This widens the operating range of the motor to the high speed exceeding the base speed.
However, in the conventional case in which the motor is continuously operated using the phases free from failure of electric current supply as the electric current flowing phases after detection of such failure, the motor might be rotated in a reverse direction if the field weakening control is performed. Specifically, the conventional case employs d-axis noninteracting control in which the d-axis electric current command value is set to “zero”. If feedback control using a negative d-axis electric current command value is performed in a two-phase drive mode (failure of electric current flow in the U phase with normal electric current flows in the V and W phases) as illustrated in FIG. 5, a q-axis electric current is generated in a negative direction as illustrated in FIG. 6 (as indicated by the hatched areas α of the graph). As a result, assist force may be generated in the direction opposite to the direction in which the steering wheel is manipulated.